1. Field of the Invention
The present invention relates to the use of optical amplifiers. More particularly, the present invention relates to a gain-clamped semiconductor optical amplifier using Raman amplification principle in which a Raman amplifier and a semiconductor optical amplifier are monolithic integrated with each other.
2. Description of the Related Art
In an optical communication system having transmitters and receivers, and fiber etc., the signal light emitted from a transmitter suffers from a transmission loss. As a result, the signal arriving at a receiver has less power than the signal originally transmitted. If the case is such that a signal arriving at the receiver has a power reduced below a threshold value, it may be impossible to perform a normal optical communication because of a receiving errors. Therefore, it is known to arrange optical amplifiers between the transmitter and the receiver so as to amplify a signal light. Thus, optical amplifiers compensate for at least a portion of the transmission loss of the signal light transmitted through the optical transmission line.
In addition, the use of optical amplifiers dramatically increases the transmission distance without optical electrical conversion.
Optical amplifiers used for the above-mentioned purpose typically comprise one of an erbium-doped fiber amplifier (EDFA), a Raman amplifier, and a semiconductor optical amplifier (SOA).
The EDFA, which uses an optical fiber doped with the rare-earth elements (for example, Erbium) for amplification, features high gain, low noise figure (NF), and high saturation output power, thereby having been widely used in a backbone network or in a metro network. However, the EDFA has drawbacks in that the costs associated with this particular amplifier is high. Moreover, the operational wavelength of an EDFA is limited to the 1.5 μm band.
However, the gain spectrum of SOA could be changed from 1.1 um to 1.6 um by the control the band gap of gain material. The semiconductor optical amplifier has advantages in that it has a small size of a few cm and doesn't need a high-priced pumping light source.
FIG. 1 illustrates the gain characteristics of a gain-clamped semiconductor optical amplifier (GC-SOA) according to the prior art. The GC-SOA has excellent gain characteristics and saturation output power characteristics.
However, as shown in FIG. 2, the gain-clamped semiconductor optical amplifier also has a very high noise figure, up to 8 dB, thereby having a limit in application in a metropolitan area or access area.
Finally, there is the Raman amplifier uses the Stimulated Raman Scattering (SRS) in an optical fiber. The Raman amplification method is a method for amplifying an optical signal by using a so-called SRS, in which a pumping light, which is a strong light, is incident into an optical fiber, to thereby cause a gain to appear on a longer wavelength side distanced about 100 nm from the wavelength of the pumping light by SRS. Subsequently, a signal light of the above wavelength band, in which the gain appears, is incident into the excited optical fiber, thereby amplifying the signal light. The Raman amplifier has an amplification band which can be controlled with comparative ease by properly setting the wavelength of the pumping light for Raman amplification, and features low noise figure.
The Raman amplifier also has drawbacks in that not only does this type of amplifier have a very low optical amplification efficiency, but the Raman amplifier also needs a high-priced pumping light source. In addition to the increased costs introduced by requiring a high-priced pumping light source, there is a problem with regard to that of size, as the whole optical amplifier module size is increased. In order to overcome the weaknesses of the different prior art optical amplifiers, technologies combining the semiconductor optical amplifier and the Raman amplifier have been recently proposed.
FIG. 3 illustrates an example of the construction of an optical amplifier in which a semiconductor optical amplifier (SOA) and a Raman amplifier according to the prior art.
The optical amplifier 100 comprises: a Raman amplification section 110 including a first optical isolator 111, a single-mode fiber (SMF) 112, a wavelength division multiplexing (WDM) coupler 113, and a pump laser diode 114; and a semiconductor optical amplification section 120 including a semiconductor optical amplifier 121 and a second optical isolator 122.
The operation principle of the optical amplifier shown in FIG. 3 will be explained as follows. First, when a 1470 nm pumping light by the laser diode 114 is injected in the reverse direction through the WDM coupler 113, an optical signal of 1560 nm wavelength band inputted through the input terminal is amplified by the Raman scattering phenomenon generated in the single-mode fiber 112. The optical signal, which is amplified by the backward-pumped Raman amplifier, is then input into the semiconductor optical amplifier 121, so as to be sufficiently amplified, and then is output through the second optical isolator 122. As described above, an input signal undergoes a Raman gain by the Raman amplification section 110 located in the front end of the semiconductor optical amplification section 120, thereby decreasing the noise figure of the semiconductor optical amplifier 121 as much as the gain.
Similar to the requirements of operating a single Raman amplifier, the hybrid optical amplifier, which is composed the Raman amplifier and a semiconductor optical amplifier, must use a high-power pump laser diode. Accordingly, the use of the laser pump makes it very difficult to reduce the size of the optical amplifier and to produce low-cost optical amplifiers. Furthermore, as the conventional optical amplifier drives two active elements, there is an additional disadvantage in that the power consumption is large.